Methods of quick and accurate, non-contact parametric measurements have been increasingly important in recent years, especially with the demands of high-volume semiconductor manufacture. As the technology required to manufacture the intricate designs of today's semiconductors has been pushed to its limits, maintaining environmental conditions has become essential for maintaining yields and producing reliable product. Even small changes in the production environment, including such parameters as humidity, temperature, pressure, and particulate count at a crucial time can significantly affect product yield and reliability.
While strict environmental control is crucial during the manufacture of semiconductors, there is a great emphasis on production volume as well. Higher volumes translate into lower unit cost. Speed and accuracy are often at odds, and finding the most profitable balance can be of utmost importance in maintaining a viable, cost-competitive product. As volumes of current generation chips produced by a typical manufacturing plant approach several million per month, slicing fractions of a second off per unit manufacturing times can increase total throughput significantly. In an ideal situation, two or more production steps can be accomplished in parallel.
These high volumes also mean that a small percentage increase in product yield, such as might be found with more accurate control of environmental conditions, greatly increases the number of functioning units. Finding measurement techniques which are better suited for a production environment, which can be used in parallel with other production steps, assists in attaining this goal.
Various problems exist with present methods of temperature measurement. Many steps of the wafer fabrication process, such as plasma etch and ion deposition, are incompatible with many commonly used temperature measurement techniques and prevent them from being used in situ.
Pyrometry has been used with some success in the manufacture of semiconductor wafers. The calibration technique (heating a sample with a known melting temperature until it liquifies and setting the temperature output by the pyrometer according to that temperature), however, is difficult, tedious, and fairly inaccurate. It has been estimated that in some systems, errors of up to 100.degree. C. can occur.
Measuring the temperature using a thermocouple, although a fairly accurate method, is impractical for a high-volume wafer manufacturing facility as it requires that the thermocouple be welded onto the surface of the wafer
The literature describes various methods of parametric measurements of a sample object which incorporate the photoacoustic effect. Methods employing the photoacoustic effect typically use a laser beam modulated at a fixed known frequency to set up a periodic heating on the surface of a sample object, such as a semiconductor. The rapid heating and cooling on the surface of the sample causes the surface to expand and contract, thereby setting up a vibration (acoustic wave) within the sample.
In some methods using the photoacoustic effect, the vibration of the sample is measured, for instance with piezoelectric means or a microphone, and the results are compared to a look-up table calibrated from measurements on like samples.
In other methods, an acoustic wave is reflected off a sample vibrated by photoacoustic means. The preperturbated wave frequency is compared to the frequency of the wave after it has reflected off the vibrating sample. The results, as in the first method, are compared to a look-up table calibrated from the measurements on like samples.
The literature describes the measurement of various parameters of an object using the photoacoustic effect including object thickness, density, and structural integrity.